formula that applies on grouped result

Hi,

I have a saved search that is grouped by department and then is summed by regular price and net price. I then want a last column that has ((reg.price-net.price)/regprice)) as a percentage to show how much discount the department has given during the last month.
Using average will not give me what I want since it will calculate the average  of each of the discount values which would be skewed since low prices item may have lesser discount then higher priced items.

ideally I'd need:
select dep, ((reg-net)/reg) as discount
from (select dep, sum(reg), sum(net) from xxx group by dep)